This invention relates to a printed antenna for microwave transmission and reception.
The printed antenna in which antenna elements and a transmission line are formed on a printed hoard has many advantages that it can he thin, light and small, it can he made in mass-production, and it can be formed integrally with electronic circuits. Such a printed antenna is used as an antenna for microwave transmission and reception of satellite broadcast, portable communication device or the like. There are various types of printed antennas. One type of printed antenna, has a linear strip antenna element having its length approximately half the wavelength of signal to be transmitted and received is used and a window is provided in a grounded conductor by cutting out the same to obtain a broad or wide band. This type of antenna is stable for its operation since it has only one resonance mode, compared with another type of printed antenna utilizing a patch type of element.
The window provided for widening a frequency hand of a strip antenna element is wide in its width and has a length more than half the wavelength of a signal to be transmitted and received. Since the window also functions as a slot antenna element, which is not desirable, the radiation from the window becomes a source of spurious radiation for a main polarized wave of the strip antenna element. As a means for suppressing the spurious radiation, the provision of projecting portions of the grounded conductor within the window at the center portion in the lengthwise direction of the strip antenna element was proposed by the same inventor in U.S. Pat. No. 5,317,324. Furthermore, the radiation from the window is reduced by spacing the end of the window away from the transmission line. In such a case, the provision of a projecting portion on the transmission line in the position of the strip antenna element was proposed by the same inventor (Japanese Patent Application 336525/1990).
When the strip antenna element provided with the window constructed so that spurious radiation is suppressed by the above-mentioned means is combined with a slot antenna element and the difference in phase of supplement of power between these linear elements are disposed 90.degree., the electric field has a phase difference of 90.degree. in time and at the same time constitutes a combination of spatially crossed oscillating electromagnetic fields to effectively radiate a circularly polarized wave since the electric field radiated from the strip antenna and the electric field radiated from the slot antenna are spatially perpendicular to each other. Although the explanation of the antenna is directed to a transmitting antenna, it should he understood that the transmitting antenna can also he used as a receiving antenna due to duality of electromagnetic field.
FIGS. 8 through 10 show a conventional printed antenna constructed by a combination of the above-mentioned linear elements for radiating circularly polarized wave. Referring to these FIGS. 8 through 10, FIG. 8 is a general plan view showing a single unit of a printed antenna constructed by using a conventional window provided with a strip antenna element, FIG. 9 is a cross-section view taken along line 9--9 of FIG. 8 showing the printed antenna together with a reflector p late, and FIG. 10 is a plan view showing a main portion of the conventional printed antenna constructed by a plurality of units of the printed antenna for radiating a circularly polarized wave by a combination of linear elements.
In FIGS. 8 through 10, reference numeral 10 indicates a strip antenna element (strip conductor), 12 indicates a window, 13 indicates an input and output portion, 14 indicates a transmission line, 15 and 16 indicate insulator substrates, 18 indicates a grounded conductor, 19 indicates a reflector plate, 20 indicates a slot antenna element, 21 indicates projecting portions in the window, and 22 indicates a projecting portion of the transmission line. The projecting portions within the window 12 are provided for suppressing spurious radiation from the window, and tapered portions are provided window near the transmission reducing spurious radiation from the window by spacing portions of the end of the window away from the transmission line 14 to weaken the connection between the window and the transmission line. Furthermore, the projecting portion 22 of the transmission line is provided for spacing the whole end of the window away from the transmission line. Explanations of the functions of these conventional printed antennas are made in the above-mentioned U.S. patent(s) and are incorporated herein. To summarize, in the following brief description is given:
Firstly, an explanation on the width of the window will he made. As the width of the window is wider, the property of the strip antenna element is expanded wider in a frequency band. However, in order to suppress the spurious radiation to a low level, it is desirable that the width of window should be narrow. Since the width of the window at the center portion thereof does not substantially affect the frequency property of the strip antenna element, the projecting portions are provided within the window for substantially narrowing the width of the window while maintaining the frequency property of the strip antenna element to be wide, and thus is capable of suppressing spurious radiation from the window.
Secondly, an explanation on the projecting portion of the transmission line will be made. The shorter the distance between the transmission line and antenna element is and the wider the superimposed portion thereof is, the closer the connection between the transmission line and the antenna element is. In general, the closer the connection is, the stronger the radiation from the antenna element is. However, with the strip antenna element disposed perpendicular to the transmission line, the area of the superimposed portions of the strip antenna element and the transmission line becomes at most the width of the strip antenna element times the width of the transmission line. When the transmission line is provided with the projecting portion, the connection between the transmission line and the strip antenna element can be closer since the superimposed portions of the transmission line and the strip antenna element become wider. Furthermore, with the transmission line provided with the projecting portion, the transmission line can be disposed to be spaced away from the end of window to reduce spurious radiation from the window, compared with the transmission line which is not provided with a projecting portion.
For the antenna for use in a portable communication device constructed by a few number of elements or the antenna for use in a satellite broadcasting transmission and reception constructed by a sub-array comprising a few number of elements, it is required to connect the antenna elements and the transmission line closely. However, with the conventional antenna in which the strip antenna element is provided with the window and is disposed to be perpendicular to the transmission line, and the transmission line is provided with projecting portion for obtaining close connection, the radiation property of the antenna element does not simply correspond to input impedance property and thus it is difficult to design the antenna element. Particularly, it is difficult to design an antenna for circularly polarized wave comprising a combination of a linear strip antenna element provided with a window and a linear slot antenna element. Furthermore, in case where the connection between the strip antenna element and the transmission line is made to be close, the superimposed portions of the window and the transmission line are wider than the superimposed portions of the strip antenna element and the transmission line, which results in greatly increased spurious radiation from the window. Furthermore, under such a condition, as the projecting portion provided within the window is intended to suppress spurious radiation, the position where the projecting portion is provided within the window, that is, the electric center of the strip antenna element is not consistent with the physical center of the length of the strip antenna element. Consequently, it is difficult to design the projecting portion within the window required for close connection with the transmission line.
In the meantime, FIG. 10 is a plan view showing a conventional printed antenna for radiating circularly polarized wave by a combination of linear elements. On designing the antenna elements using a plurality of pairs of elements, the connection between the strip antenna elements and the transmission line is not required to be close. Under such a condition, in case where the transmission line is provided with projecting portions, the correspondance between the radiation property of strip antenna element and the input impedance property is relatively easily obtained. Furthermore, in case where the transmission line is not provided with the projecting portions, spurious radiation from the window is suppressed to a very low level by providing the projecting portion within the window in the physical center position of length of strip antenna element since a desired connection between the strip antenna element and the transmission line can be obtained under a condition that the window and the transmission line are not closely superimposed. Thus, in case where a close connection with the transmission line is not required, the strip antenna element with little spurious radiation can be realized by adoption of the projecting portion of the transmission line or the projecting portion of the window, different from that of the case where a close connection is required. That is, the property of a single element required for a close connection and the property of plural elements not required for a close connection are quite different from each other. It should be understood that the design for the antenna using a sub-array comprising a multiplicity of elements not required for a close connection can be easily made, if the design data obtained by a single element is easily applicable to the design for plural elements.